My wiggly rear end

[ShadeD1]

happened to me at MSR Cresson last year. It was brake bias, my front rotors were shot.

[AMPowerJ]

I am new to this whole tracking thing I will admit, but wouldn't you expect the rear to become much less planted and stable under heavy braking because of the extreme weight transfer to the front tires? It doesn't seem like any alignment could overcome physics and the loss of the size of the contact patch on the rear tires under heavy braking.

I was at CMP yesterday and I felt this too under heavy braking. I never felt like I was going to lose the back end but I could feel it squirm. I certainly understand why you only want to brake this way when you are going straight. Any turning and you would spin easily.

My instructor told me the same thing that rldzhao mentioned. Brake hard early, roll out of the brake to settle the car and transfer the some of the weight back to the rear to balance the car then roll completely off the brake, turn, and power into and through the turn. I wasn't at the level where I was trying to trail brake.

[jml]

There are two places at TWS this happens to my E90, and it's because there is a tiny bit of lateral acceleration or other input going into the car like a large suspension oscillation. Running stock suspension and stock brakes, but 275 NT01s and track pads, 0.7 toe in the rear.

The first is braking on the low line into T1, which is at the end of a very long highly banked section of the Nascar oval as you transition into the interior road course. You are turning a slight amount and track is bumpy, 140-150mph.
The second is in the opposite direction, braking for T15 at the end of a long straight on the nascar oval. The oval isn't actually straight, so the tires are spinning at different speeds. You brake from 160 to 100.

The feeling is the car wiggling side to side, like it's starting a very light tank slapper. A couple times it has scared me. I could get away with this in the E46 for some reason, but my cure for the issue is to make sure *heavy* braking is done when the car is settled (minimal acceleration on any axis (except the positive Z-axis - compression).

[klammer]

Do you all feel this in the steering wheel? I've been feeling a bit of steering wheel vibration under heavy braking (100+ down to 20-30), but may just be me needing to straighten up first.

[bigjae1976]

I thought the moon shaped toe link in the rear suspension is one of the major causes of rear looseness under braking? It is designed to flex so there sre significant changes in rear toe.

At least that's what Dinan sells. I've seen some other tuners advertise the same thing for a similar product.

Originally Posted by jml

The first is braking on the low line into T1, which is at the end of a very long highly banked section of the Nascar oval as you transition into the interior road course. You are turning a slight amount and track is bumpy, 140-150mph.
The second is in the opposite direction, braking for T15 at the end of a long straight on the nascar oval. The oval isn't actually straight, so the tires are spinning at different speeds. You brake from 160 to 100.

The feeling is the car wiggling side to side, like it's starting a very light tank slapper. A couple times it has scared me. I could get away with this in the E46 for some reason, but my cure for the issue is to make sure *heavy* braking is done when the car is settled (minimal acceleration on any axis (except the positive Z-axis - compression).

I usually stay along the outside lane to lengthen the braking zones. I remember going CW was much rougher.

On the otherhand, going CCW is much scarier. I always have thoughts of that ditch.

[paradocs98]

I'm glad this has generated such a response. Lots of interesting thoughts. I think a lot of it probably has to do with me and improving my braking/turn-in technique, so the plot of brake force vs steering angle was very helpful. I'm intrigued about the Dinan rear toe links. I had seen those on their website before and wondered how useful they would be. They might actually be of some value.

[Onurleft]

I have a feeling this might have something to do with your BBK. I have read several things about BBK's and the factory ABS control module not reading properly. I'd give Turner Motorsports a call, i'm sure this is a common problem

[bigjae1976]

Originally Posted by paradocs98

I'm glad this has generated such a response. Lots of interesting thoughts. I think a lot of it probably has to do with me and improving my braking/turn-in technique, so the plot of brake force vs steering angle was very helpful. I'm intrigued about the Dinan rear toe links. I had seen those on their website before and wondered how useful they would be. They might actually be of some value.

Here is the info from Dinan's website...

Dinan’s high performance rear toe link adds precision and predictability when the M3 is driven to the limit.

The stock rear toe link is built with rubber bushings and a curved foraged alloy rod which is designed to flex. During hard acceleration this flex allows the rear wheels to have excessive toe change under hard acceleration and braking. All of this flex in the stock toe link causes the M3 to be unstable.

Dinan stops the flex by replacing the rubber bushings with Teflon coated machined steel rod ends and the OEM curved bar with a straight billet aluminum rod, thus adding stability. The Dinan rear toe link also incorporates a hex nut shaped center to aid in the necessary fine adjustment of the toe link that is tough to achieve using the stock link

[lucid]

It is true that the deflection of the OEM rubber suspension bushings under heavy loads might result in dynamic toe changes, and I can only imagine that there is room for improvement there if one doesn't mind stiffening things up on a DD. However, I have a hard time believing the issue can be so severe that the car becomes significantly unstable. Why would BMW release an inherently unstable M platform (what would happen if one were to do an emergency stop on the autobahn at 140mph)? Sure, I have felt the rear end wiggling around a bit in certain braking zones under hard braking on this car at times, but I did not perceive it to be a major issue--something one lives with given it is a street car. People have been tracking this car for over 2 years now, and I am not aware of any history of complaints on this. Having said that, the OP's suspension is not stock, which opens up many possibilities/interpretations...

[dcstep]

Do you have 18x10" will with 25 offset? If so, I think it may be screwing with your DSC. I turn it off when I AX, but on the street I feel the DSC interacting with the rear brakes at the wrong times when I drive even a little agressively. It'll touch the inside rear brake before the rear end has broken away. I'm running an APEX ARC-8 10x18" wheel with Dunlop Direzza Star Spec 275/35-18 and noticed the change in the way the rear behaved right away.

Try running at least a few laps with DSC off (within your limits, obviously) to see if there's a difference. Your wheel offset and, perhaps, your Stoptechs, may be screwing up the DSC's little mind.

Dave

[bigjae1976]

Originally Posted by dcstep

Do you have 18x10" will with 25 offset? If so, I think it may be screwing with your DSC. I turn it off when I AX, but on the street I feel the DSC interacting with the rear brakes at the wrong times when I drive even a little agressively. It'll touch the inside rear brake before the rear end has broken away. I'm running an APEX ARC-8 10x18" wheel with Dunlop Direzza Star Spec 275/35-18 and noticed the change in the way the rear behaved right away.

Try running at least a few laps with DSC off (within your limits, obviously) to see if there's a difference. Your wheel offset and, perhaps, your Stoptechs, may be screwing up the DSC's little mind.

Dave

Is this caused by the non-staggered OD between the front and rear tires?

[dcstep]

Originally Posted by bigjae1976

Is this caused by the non-staggered OD between the front and rear tires?

Don't know, but that's a good theory.

[lucid]

Just to update the thread on the E30 M3 instability under braking issue I brought up...

It seems to have been resolved after a couple of changes:

1. I installed Wilwood calipers and rotors on all 4 corners. That might have solved an issue with an OEM caliper that needed a rebuild. That also moved the bias further back.

2. I deleted the parking brake. While I was removing the parking brake shoes, I discovered that one of the linings had come loose from one of the shoes, so perhaps that was getting jammed in the system under hard deceleration.

[dcstep]

Thanks for the follow-up report Lucid. That makes a lot of sense.

[lucid]

Too bad I had to make the changes in conjunction and we can't pinpoint the cause. Now that I think about it, I also installed a new 4-clutch 30/90 diff. The old unit was really beat. Who knows if it might have been locking up somehow at the wrong time--unlikely but still...

[paradocs98]

And for follow-up from me, I did a PCA event at NJMP Lightning last Friday. I did notice the rear-end wiggle again under hard braking, especially at the end of the main straight into turn 1, but it's not unmanageable. I do think part of it has to do with my technique--being too aggressive with trail-braking or starting to turn-in too soon certainly worsens the feeling of instability. But the bit of wiggle that occurs even under pure straight-line hard braking didn't overly concern my instructor--he felt that to a certain degree it's expected.

[leo95se]

I noticed this when I changed to a square setup with nt-01s. I think it's the increased front grip and possibly softer sidewall than street tires. definitely a softer tire, which may create some squeem along with increased grip.